The present invention relates to a method and an apparatus for the gasification of coal with oxygen or oxygen-containing gas and water vapor and also if necessary CO.sub.2, and more particularly to such a method and an apparatus in which powdered coal is gasified in at least one burner, e.g. a cyclone burner, with oxygen or oxygen-containing gas and steam, and also if required added CO.sub.2, and the primary gas thereby produced is passed upwardly, preferably in a countercurrent flow and under pressure through coarser coal contained in a shaft gasifier, e.g. a bed of lumps of coal having upper and lower free surfaces, thereby producing product gas and forming liquid slag.
In the present connection, coal is to be taken as meaning the various fuels which contain free carbon, such as anthracite, bituminous coals, brown coal, soot, briquettes, for example. Liquid or gaseous fuels may also be used instead of the fine or powdered fraction. A product gas containing carbon monoxide and hydrogen is produced by the gasification operation. Depending on its composition in each case, a gas of this type may serve as a fuel gas for use in fuel cells or for synthesizing ammonia, methanol, hydrocarbons, phosgene and oxo-alcohols, for example.
With autothermal gasification, on which the present invention is based, there is a combination of the gasification of coarser coal, preferably lumps of coal under increased pressure, preferably in the solid bed in a countercurrent direction, with the gasification of coal dust under increased pressure in a direct current. Measures are taken to ensure that the operation takes place in a temperature range above the ash melting point and that the slag is drawn off from a shaft gasifier which has no grid in the liquid state. The coarser coal, which is preferably present as a bed of lumps of coal, thus assumes the function of a cooling and filtering unit for the hot primary gas supplied into the lower part of the shaft gasifier. The process heat requirement is met by the partial combustion of the coal dust with oxygen.
There are three known principle processes for the autothermal gasification of coal:
1. Fluidized current gasification using finely ground powdered coal and producing a gas having a high temperature and a low proportion of methane;
2. Fluidized bed gasification using coal of medium grain size and obtaining medium gas temperatures; and
3. Shaft gasification using lumps of coal and producing a gas of lower temperature and, unless coke is used, a high proportion of methane.
As a result of the poor heat economy of fluidized dust gasification and the sensitivity of a shaft gasifier relative to fine coal, various combinations of such methods have already been proposed. For example, a method of gasifying coal is known from German DE-PS No. 4 58 879 in which the coal is separated into lumps and dust by sieving. The lumps are supplied to a shaft gasifier, while the coal dust is gasified in a burner, and the primary gas thereby produced is conducted into the shaft gasifier for drying and gasifying the lumps of coal. The liquid slag collects on an inclined base of the shaft gasifier in front of a lower free sloping surface of a bed of the lumps of coal and may be discharged therefrom via a slag outlet. The process may be controlled in a manner known per se by the injection of water vapor. In such process, however, discharging of the slag creates a problem, especially if the process is carried out under pressure. Such method is also uneconomical, because external heat is required for supplying the water vapor.
Improving the heat balance of a gasification method is known per se from German DE-PS No. 2 88 588, by quenching slag discharging at a lower part of a shaft gasifier inside the shaft gasifier, and by granulating the slag in a water bath. In this connection, the liquid slag is first collected in a tank and passed therefrom into the water bath disposed beneath the shaft gasifier. Steam produced when the slag enters the water bath is compressed via a bypass line into the upper part of the shaft gasifier above the slag fusing zone. This is to prevent the steam from reaching the lower part of the shaft gasifier. In this method inadequate use only on the heat content of the liquid slag is possible, since the steam produced is not satisfactorily used as process steam.
A slag bath generator is known from Chem. Ing. Technik, 1956, No. 1, pages 25 to 30, in which pulverized fuel and a gasification agent are injected through separate nozzles into a shaft gasifier obliquely downwardly and approximately tangentially at the level of a slag overspill or overflow located at the base of the shaft gasifier. The overflowing slag reaches a water bed arranged beneath the base of the shaft gasifier and is therein granulated. Steam supplied for the gasification operation must be produced separately.
According to German DE-PS No. 10 42 817, in which primary gas supplied by two lateral dust gasifiers is passed through a bed of coal or coke in a shaft gasifier, the coal dust must be extensively reacted with oxygen before it meets the bed since otherwise the bed would clog. In this method, the ash may be drawn off in the liquid state or the dry state.
Drawing off liquid slag from shaft gasifiers which are under pressure requires complicated equipment. This means that none of the previously mentioned methods are suitable for gasification under pressure. Also, with the known methods, the considerable content of sensible and latent heat of the liquid slag is partially or completely lost.
A method of producing fuel gas mixtures from fine-grained fuels is known from German DE-PS No. 9 08 516, wherein some of the coal is burnt in burners, preferably constructed as cyclone burners, with oxygen and steam as gasification agents, and the primary gas thereby produced passes through a fluidized bed consisting of the remaining coal, thus resulting in chemical reactions taking place with the coal and in cooling of the primary gas. This method combines the relatively high space-time yields of a direct current method in the first stage with the efficient utilization of heat of a countercurrent method in the second stage. However, such method may be used in paractice only if the burners produce a dry ash, since otherwise the fluidized bed would coalesce.